Field of the Invention
The invention relates to a drying station for drying printed sheets and a drying method for use in such a drying station.
In printing processes, it is generally desirable to dry the printing ink applied to the printing material as quickly as possible, in order to avoid smudging. In connection therewith, it has already become known heretofore to accelerate the drying method by irradiation, in particular with ultraviolet or UV light. Although this procedure delivers very good results with regard to the drying, it also entails disadvantages, in particular when printing sheets. The sheets are conveyed past the irradiation device, such as a UV radiator, for example by a transport belt. This causes the transport belt to be exposed to irradiation with UV light in the context of the drying process, because the sheets do not cover the transport belt completely. With regard to plastic material from which the transport belt is generally produced, the action of the UV radiation causes accelerated aging, i.e., the lifespan of the transport belt is reduced, so that the transport belt has to be replaced prematurely. Due to the shortened time interval before the transport belt has to be replaced, higher maintenance costs arise, and the number of machine stoppages is increased.
In order to reduce the UV loading of the transport belt, it would be possible to try to operate the irradiation device cyclically, so that irradiation is performed only when a sheet is just being guided past the irradiation device. In contrast, when the gaps present between successive sheets pass the irradiation device, the irradiation device is switched off. This procedure has a drawback, however, in that switching the irradiation device on and off is very complicated in technical terms and would have to be synchronized very precisely with the sheet transport. Furthermore, the irradiation device would have to be focused very precisely parallel to the direction of movement of the transport belt, on the one hand, to ensure complete irradiation of the printed region of the sheets and, on the other hand, to prevent undesired irradiation of the transport belt as completely as possible.
In principle, it is also possible to produce the transport belt from a material which is not impaired by the UV radiation, or impaired thereby only to a tolerable extent. For example, a metal fabric could be used. This has a disadvantage, however, in that the use of such materials entails very high costs, and/or the characteristics of the plastic materials which are to be replaced by these metal materials can be simulated only rather inadequately.
It is therefore an object of the invention to permit, with a tolerable expenditure, the application of irradiation techniques for accelerating drying in the case of sheets which are conveyed by a transport belt, and more specifically, to provide a drying station and a method for drying printed sheets and a printing machine having a drying station.
With the foregoing and other objects in view, there is provided, in accordance with one aspect of the invention, a drying station for drying printed sheets transported by a transport belt, comprising an irradiation device for irradiating the printed sheets, a deflection section located in vicinity of the irradiation device for guiding the transport belt away from the irradiation device, and a shielding device disposed between the transport belt and the irradiation device for shielding the transport belt against radiation from the irradiation device while the sheets are exposed to the radiation.
In accordance with another feature of the invention, the deflection section has such dimensions that the sheets are lifted off the transport belt, at least in some regions, at a beginning of the deflection section.
In accordance with a further feature of the invention, the shielding device is formed with a chamfer.
In accordance with an added feature of the invention, the drying station further comprises devices for guiding and for transporting the sheets, the guiding and the transporting devices being disposed on a side of the shielding device facing towards the irradiation device.
In accordance with an additional feature of the invention, the drying station further comprises blower nozzles disposed on the shielding device for guiding and for transporting the sheets.
In accordance with yet another feature of the invention, the drying station further comprises air ducts disposed in the shielding device and serving for supplying air to the blower nozzles.
In accordance with yet a further feature of the invention, the drying station further comprises cooling ducts disposed in the shielding device and being traversible by a coolant flow.
In accordance with yet an added feature of the invention, the shielding device has a plate-like construction.
In accordance with yet an additional feature of the invention, the shielding device is formed of three layers made up of a first layer facing towards the irradiation device, and being provided with blower nozzles and cooling ducts, a succeeding second layer serving as a closure plate for the cooling ducts which are open towards the second layer, and a third layer provided with air ducts therein which open towards the second layer.
In accordance with still another feature of the invention, the second layer of the shielding device is formed with boreholes through which air is able to pass from the air ducts of the third layer to the blower nozzles of the first layer.
In accordance with still a further feature of the invention, the drying station further comprises at least three deflection rollers disposed in the deflection section.
In accordance with still an added feature of the invention, the irradiation source is a UV radiator.
In accordance with another aspect of the invention, there is provided a printing machine having a drying station for drying printed sheets transported by a transport belt, comprising an irradiation device for irradiating the printed sheets, a deflection section located in vicinity of the irradiation device for guiding the transport belt away from the irradiation device, and a shielding device disposed between the transport belt and the irradiation device for shielding the transport belt against radiation from the irradiation device while the sheets are exposed to the radiation.
In accordance with a further aspect of the invention, there is provided a method for drying printed sheets, which comprises conveying the sheets by a transport belt into the vicinity of an irradiation device, and subjecting the sheets to irradiation in order to accelerate drying thereof, and further comprising separating the sheets from the transport belt, at least in some regions, before the sheets are guided past the irradiation device, guiding the transport belt, on the one hand, and the sheets, on the other hand, past the irradiation device separated from one another, and subjecting only the sheets to the irradiation, while shielding the transport belt from the irradiation.
In accordance with an added mode, the method of the invention further comprises, in the vicinity of the irradiation device, guiding the transport belt over a deflection section.
In accordance with an additional mode, the method of the invention further comprises, in the vicinity of the irradiation device, guiding the sheets over a shielding device which serves for shielding the transport belt from the irradiation.
In accordance with a concomitant mode, the method of the invention further comprises guiding the transport belt and the sheets together again after passing the irradiation device.
The drying station according to the invention for drying printed sheets which are transported by a transport belt thus has an irradiation device for irradiating the sheets. Arranged in vicinity of the irradiation device is a deflection section, which guides the transport belt away from the irradiation device. Arranged between the transport belt and the irradiation device is a shielding device, which shields the transport belt against the radiation from the irradiation device without shielding the sheets therefrom.
This arrangement offers the advantage that the radiation acts upon the sheets and, as a result, very quickly dries the printing ink applied to the sheets. At the same time, the transport belt is prevented from likewise being exposed to the irradiation with consequent triggering of undesired aging effects. It is thereby possible, with regard to the transport or conveyor belt, to have recourse to a tried and tested plastic belt while, nevertheless, utilizing the advantages of accelerating the drying by irradiation.
The deflection section is advantageously dimensioned so that the sheets, due to the stiffness thereof, are lifted off the transport belt, at least in some regions, at the beginning of the deflection section. An added device for separating the sheets from the transport belt is therefore not required.
In order to facilitate the transfer of the sheets from the transport belt to the shielding device, it is advantageous if the shielding device is formed with a chamfer in the region thereof wherein the sheets engage or make contact with the shielding device.
It is possible to dispense with a separate device for guiding and for transporting the sheets in the region or vicinity of the irradiation device, because these functions can also be performed by the shielding device.
In the region or vicinity of the shielding device, the guidance and the transport of the sheets are performed by blower nozzles. These can be implemented or realized with little effort, and operate very reliably, because there are no moving parts. Only a compressed-air connection is necessary. Within the shielding device, the air is supplied to the blower nozzles via air ducts.
In addition to the air ducts, the shielding device also has cooling ducts, through which a coolant flows, so that a constant temperature of the shielding device is assured. This offers the advantage that the shielding device, in addition to the shielding function thereof, also prevents impermissible heating of the sheets and of the transport belt. As a result, an irradiation device with a high power density can be used, which permits a high throughput.
In a preferred exemplary embodiment, the shielding device is of plate-like construction and is formed with three layers, namely, a first layer which faces towards the irradiation device provided with the blower nozzles and the cooling ducts. A succeeding second layer serves as a closure plate for the cooling ducts of the first layer, which open towards the second layer. A third layer has the air ducts therein which open towards the second layer. In addition, the second layer is formed with boreholes through which air passes from the air ducts of the third layer to the blower nozzles of the first layer. This construction has the advantage that it may be produced with a tolerable expenditure and, if the mutually joined individual layers are separable, the air ducts and the cooling ducts are very easily accessible for maintenance work.
In the preferred embodiment, the deflection section has three deflection rollers. However, there may also be more deflection rollers, depending upon the conditions of the respective application thereof.
The radiation source is preferably constructed as an ultraviolet or UV radiator, because a wide spectrum or range of UV-curing printing inks is available and the drying takes place very quickly.
In the method according to the invention for drying printed sheets, the sheets are transported into the vicinity of an irradiation device by a transport belt. Before the sheets are guided past the irradiation device, they are separated, at least in some regions thereof, from the transport belt. The transport belt and the sheets are guided past the irradiation device separated from one another, and only the sheets, but not the transport belt, are subjected to the irradiation.
The separate handling of the sheets offers the advantage that the transport belt can be protected very reliably against the action of the irradiation.
In the vicinity or region of the radiation device, the transport belt is guided over a deflection section. This offers the advantage that the transport belt is guided past the radiation source at a greater distance therefrom than the sheets, and that space for the installation of further components is created.
Because the transport belt and the sheets are guided together again after passing the irradiation device, the flow of the method is not interrupted, in spite of the separate handling of the sheets in the vicinity or region of the irradiation device.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a drying station and a method for drying printed sheets and a printing machine having a drying station, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.